1. Field of the Invention
The present invention relates to a picture data reproducing apparatus and method, usable suitably for reproducing picture data compressed by encoding according to the MPEG (Moving Picture Experts Group) standard by decoding the data.
This application claims the priority of the Japanese Patent Application No. 2002-206120 filed on Jul. 15, 2002, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Recently, various digital video coding techniques as typified by MPEG-2 (ISO/IEC 13818) have been proposed to compress a video by coding. The video compression according to the MPEG standard is such that a signal produced by effecting a hybrid transform which is a combination of inter-picture motion compensation and DCT (discrete cosine transform) undergoes quantization and variable-length coding.
According to the MPEG standard, picture planes (frames or fields) included in a video are compressed by coding them into any one of picture types including I, P and B pictures. The I picture is an intra-frame predictive coded picture or an intra coded picture. It is a picture predictively coded in one frame. The P picture is a frame forward-predictive coded picture or a predictive coded picture. It is a picture predicted by referring to a previous frame (I or P picture) which is already coded. The B picture is a bidirectionally predictive coded picture or a bidirectional coded picture. It is a picture predicted by referring to two frames, namely, a previous one and later one.
As above, according to the MPEG standard, video compression is done by inter-picture predictive coding, whereby it is made possible to efficiently compress a video and access the compressed video at random. Also, according to the MPEG standard, pictures of each of the above types are compressed into a data stream formed in units of a group of pictures (GOP) including an arbitrary number of pictures. The MPEG standard prescribes that at least one I picture should be included in a GOP to make it possible to make random access to a video compressed in units of GOP.
Reproduction of picture signals compressed according to the MPEG Standard in a conventional picture recorder/player will be explained herebelow. The conventional picture recorder/player includes an encoder and decoder for compression and expansion (decompression), respectively, of picture data according to the MPEG Standard, it records picture data compressed by the encoder to a recording medium or expands picture data read from the recording medium.
The recording medium has recorded therein data resulted from encoding of a data stream shown in FIG. 1 for example. The conventional picture recorder/player decodes the data stream recorded as shown in FIG. 1 for reproduction. The references “I”, “P” and “B” added to pictures in the data stream indicate I, P and B pictures, respectively.
To reproduce the data compressed by encoding data stream shown in FIG. 1, the conventional picture recorder/player begins with decoding a picture I0. The I picture is completely encoded in an image and can be decoded independently. Next, the conventional picture recorder/player decodes a forward predictive coded picture P1 on the basis of the decoded picture I0. Then, the picture recorder/player decodes bidirectionally predictive coded pictures B1 and B2 on the basis of the decoded pictures I0 and P1. The B pictures have been encoded by reference to a temporally preceding I or P picture. So, it is necessary to decode the pictures I0 and P1 prior to decoding the pictures B1 and B2. As above, the conventional picture recorder/player decodes the encoded data stream shown in FIG. 1 in an order of I0, P1, B1, B2, P2, B3, B4, . . .
For a so-called variable-speed reproduction in which data is reproduced at a half, quarter, . . . , of a normal speed, frame is advanced and similar operations are done, it is necessary to appropriately select an amount of data for addition to a stream buffer and a timing of data addition. Especially, when reproduction direction is shifted between FORWARD and REVERSE in the course of a variable-speed reproduction, it is necessary to display decoded picture data without skipping over any picture in the data.
According to the aforementioned MPEG Standard, the I, P and B pictures are not identical in data amount to each other. Generally, the I picture carries a greatest amount of data, P picture carries a next greatest amount of data, and the data amount in the B picture is smallest. Also, GOPs each including an arbitrary number of pictures are not equal in data amount to each other. Conventionally, an appropriate amount of picture data to be added to the stream buffer cannot easily be selected, and such picture data cannot be added to the stream buffer in a fixed timing.
When the reproduction direction is shifted from FORWARD to REVERSE, or vice versa, in the course of a variable-speed reproduction, prevention of image quality requires very complicate operations of control and thus no smooth variable-speed reproduction cannot be accomplished.